During surface grinding operation, a machine tool which commonly includes an electric motor and a grinding disc is usually used. The grinding disc holds mating sandpaper at the bottom thereof. The electric motor drives the grinding disc rotating at a high speed to allow the sandpaper to grind the surface of a targeted object. In general, the grinding disc and the mating sandpaper are formed in common sizes of 3, 5 and 6 inches. These grinding disc and sandpaper are available on the market and cost at affordable levels. The sandpaper is diversified in specification and material to meet different requirements of users and grinding operation.
However, for the larger area grinding operation (such as tabletop, wall surface, ground surface and the like), the grinding disc and sandpaper at the common dimension do not have large enough diameters to provide only limited grinding area, hence operation speed is slower and efficiency is lower. In response to surface grinding work of a larger area a machine tool providing a larger grinding area is needed. For instance, R.O.C. Pat. publication No. 339718 entitled “Disc structure for Nylon fiber grinding wheels of a larger area disc” provides a disc structure of Nylon fiber grinding wheels for a handheld grinder capable of grinding a larger area at a higher efficiency. The disc is made from plastics integrally by injection and has desired flexibility. It has a hexagonal screwing seat extended from the center of the top surface. The screwing seat has a hole run though vertically from the top end thereof to the bottom side of the disc. A hexagonal nut is embedded in the hole close to the top end. It can be easily installed and removed from the handheld grinder. Moreover, the screwing seat has a circumference at the bottom end with an annular rib formed thereon and jutting towards the disc to bear the torsional force loaded onto the grinder at high rotational speed without breaking the disc. It can perform grinding work of a larger grinding area faster.
However, the aforesaid conventional machine tool requires the grinding disc and the mating sandpaper with larger specifications but not the common specifications, they are difficult to acquire and usually are expensive even if available. Moreover, materials and specifications of the sandpaper also are varying without sufficient selections. It still leaves a lot to be desired in terms of improving grinding work efficiency of a large area and costs. There is still room for improvement.